1. Field of the Invention
This invention relates to facsimile transmission systems, and more particularly, to the efficient encoding of continuous-tone information from an original pictorial image such that an acceptable rendition of the original picture can be reconstructed.
2. Description of the Prior Art
Facsimile transmission is a costly proposition. Each image comprises a very large amount of visual information. In conventional facsimile transmission systems, images are typically divided into a matrix of picture elements (pels) of a sufficiently small size such that when each element (pel) is scanned as having only one light intensity level, an acceptable copy results. A substantial amount of visual information is lost, but the characteristics of human perception are such that an acceptable copy can still be produced. Even with this amount of visual information, facsimile still has not been economically feasible in the past due to transmission and equipment costs as well as the long intervals required for transmission. Better image quality usually results from more information being transmitted. However, when more information is transmitted, more transmisson time is required.
Used in conjunction with the technique of scanning an image as a matrix of pels are many compression techniques designed to further reduce the amount of visual information transmitted while still producing visually acceptable copies. In some techniques, the picture information has been reduced to purely white or black pels. This results in a one bit per pel (bit/pel) transmission rate.
Where the images are two-tone, i.e., black written material on a white sheet of paper, compression techniques have been developed which require a transmission rate of substantially less than one bit per pel. This is due to the larger percentage of white background, the fairly regular grouping of black pels, and the fact that only two tones of color are of significance.
However, in the reproduction of continuous-tone or gray-scale images, such as a photographic picture, with pels having many different levels of light intensity, substantial visual information that is neither white nor black is of some significance.
Two digital techniques for continuous-tone images have been developed which result in a transmission rate of one bit per pel. These two techniques have reduced visual information transmitted while still producing an adequate and approximate reproduction of the original image. These two techniques encode all the pels into black or white signals. The compression techniques for two-tone images, mentioned earlier, however, cannot efficiently reduce the bit rates of these techniques because of the increased randomness of the black and white pels generated.
One of the prior art techniques for encoding continuous-tone images is "dither processing" wherein input electrical signals are transmitted as black or white pels after being compared to corresponding position-dependent threshold values from a two-dimensional ordered dither matrix of sample points assigned different voltage values corresponding to different light intensities. The input signals exceeding the corresponding dither thresholds are transmitted as black pels while the others are transmitted as white, resulting in a transmission rate of one bit per pel. Some additional reduction has been produced with dictionaries by approximating and grouping similar groups of pels so as to send codes. This creates problems--requiring a substantial memory storage of reference data for comparison.
The other technique for encoding gray-scale or continuous-tone information, taught by Young in U.S. Pat. No. 3,294,896, is premised on the observation that in a typical picture the fine or black and white detail which form the edges or outlines are more important than the gray of shaded areas. Hence, the gray information may be conveyed with considerably less accuracy of position and extent than is required for the detail.
In Young, the input electrical signals of an image are compared to a black threshold, a white threshold, and a third threshold which adjusts periodically to a pre-established dot pattern scheme simultaneously. Where the input signals are above the black threshold, black pels are transmitted. Where the input signals are below the white threshold, white pels are transmitted. Where the input signals are "gray", they are transformed into a sequence of black and white pels with the periodically adjusting threshold for transmission.
Greater bandwidth compression of continuous-tone images than that provided by the above techniques is desired if continuous-tone images are to be economically transmitted over conventional telephone lines on a mass scale for storage or image reproduction.